Use default avatars for author and others in comments

functions.php

<?php // Do not add this to your functions.php.

/* Use default avatars for author and others in comments */
add_filter( 'get_avatar', 'agf_custom_avatars', 10, 3 );
function agf_custom_avatars( $avatar, $size ) {

	if ( is_admin() ) {

		$size   = '48';
		return $avatar;

	}

	$size   = '96';
	$author = get_comment_author();

	if ( 'Shea' === $author ) {

		$avatar_url = get_stylesheet_directory_uri() . '/images/red-icon-155.png';

	} else {

	$avatar_url = get_stylesheet_directory_uri() . '/images/bw-icon-155.png';

	}	

	$avatar = '<img src="' . $avatar_url . '" class="avatar avatar-' . $size . ' photo" width="' . $size . '" height="' . $size . '" />';

	return $avatar;
}